An electrically rewritable nonvolatile memory device (EEPROM) is an example of a nonvolatile memory device where data is not erased when power is not applied. An EEPROM may use a floating gate structure. As semiconductor devices progress towards relatively high integration, there is a need to reduce the size of a floating gate cell. However, relatively high voltage may be required in programming and erasing operations. It may be difficult to secure process margins, and may be impossible to reduce the cell size. Nonvolatile memory devices substituting a floating gate cell, such as silicon-oxide-nitride-oxide-silicon (SONOS), FeRAM, SET, NROM, etc are being researched. Among them, the SONOS cell holds promise as a next-generation cell, substituting a stack-type floating gate cell.
However, there is no a method for measuring threshold voltage for the SONOS flash cell. The characteristics of the threshold voltage with respect to the pulse width and applied voltage can not be measured so that it may be difficult to account for them in the design of the SONOS flash cell.